Anti-lock braking systems have now progressed to the point where they are standard on many vehicles. The use of integrated traction control systems is also now becoming increasingly widespread, and it is anticipated that their use will parallel that of anti-lock braking systems. These may be further integrated in other vehicle systems, i.e. power steering suspension or other ride control systems. In both braking systems rapid deployment of brake calipers or brake shoes are necessary in order to perform the intended control function. In anti-lock braking systems, when locking of the wheels due to over-application of brake pressure or loss of traction due to the nature of the surface, i.e., gravel, ice, or snow, is encountered, the automotive braking system rapidly pulsates the brakes between an off and an on condition, allowing maximal retention of braking ability while yet retaining the ability to steer the vehicle in a stable fashion. In traction control systems, loss of traction in a driving wheel is countered by a momentary application of brake pressure, thus restoring traction. In either case, high pressure hydraulic systems are desirable to affect the rapid changes necessary to achieve the desired control.
While some systems rely on the pressure generated by the brake master cylinder to achieve the desired results, response time in such systems is marginal, and thus, optimum response is not achieved. To overcome these drawbacks, high pressure pumps, eccentrically driven by an electric motor, supply the high pressure needed to actuate the system. Driving the high pressure pump at all times would be wasteful of energy and further create unwanted noise. Thus, the motor-driven high pressure pump is actuated only when the need for high pressure is sensed by the circuitry associated with anti-lock braking system or traction control system, as the case may be.
A typical anti-lock braking system is shown schematically in FIG. 1. Hydraulic fluid from the brake pedal 2 actuated master cylinder 1 flows through line 4 through normally open isolation solenoid valve 5 to brake caliper slave cylinder 6. Except for the presence of the additional normally open isolation valve, the system thus far described is similar to the normal braking system of the automobile. In an anti-lock brake system, detection of a lock condition actuates high pressure pump 7 and closes isolation solenoid valve 5. At the same time, hold/dump valve 9 is opened, allowing pressure to bleed from the brake cylinder to the low pressure accumulator 8. The brakes are thus momentarily released. To reapply the brakes, pressure from high pressure pump 7 is diverted to the brake cylinder by opening the isolation solenoid valve and closing the hold/dump valve, once again restoring braking pressure. This cycle repeats itself rapidly, resulting in rapid pulsations of on and off conditions, thus achieving maximal braking while avoiding a locked condition. During the dump cycle, and also during the period of time when the high pressure pump is outputting excess high pressure fluid, fluid flows into the low pressure accumulator. In order to minimize vibrations and to lessen the potential for damage to the system caused by them, it has proven useful to place an attenuator 11 on the outlet side of the pump between the pump outlet and master cylinder. The attenuator generally includes within its housing, a reduced diameter orifice 10, which, in combination with a compressible substance within the attenuator, markedly reduces pressure fluctuations and vibration felt at the pedal.
As the brakes are released during an ABS or other vehicle control cycle, pressurized fluid from the brake slave cylinders must flow from the slave cylinders to another portion of the system. While this flow could be directed to the master cylinder, by doing so, the master cylinder would be subject to considerable pressure fluctuations and vibration, which would then be felt by the vehicle operator. Thus, it is common to isolate the master cylinder at such times by allowing fluid to flow into the low pressure accumulator 8. With the accumulator normally selected, fluid flowing into the accumulator displaces a piston against a return spring located in the accumulator body. The tension of the spring is selected to provide a return force which is less than the brake-apply force at the slave cylinder. An outlet from the accumulator supplies fluid to the inlet of high pressure pump 7.
Typical low pressure accumulators utilize several seals, sleeves and other components, all of which must be hand-assembled. In addition, the dynamically sealed piston, in general, bears against a light alloy body. Wear of the body or the incidental occurrence of particulates may give rise to sticking of the piston and intermittent or total failure of the device. To minimize wear, a hard, heat-treated aluminum alloy must be used for the hydraulic control unit (HCU) body.
The fluid pressure within the brake line as stored by the low pressure accumulator is generally within 10 to 50 psig, as compared to brake apply pressures ranging from 0 to 3,000 psi.